The next generation liquid crystal displays (LCDs) will require LCDs with motion picture quality, ultra high contrast, brightness and wide viewing characteristics. Traditional LCDs are based on nematic technologies which are still deficient in response time for perfect motion picture viewing. The response time of LCDs has been improved by using thinner cell gap and higher switching voltage. However, the approach of using thinner cell gap increases the difficulty in the manufacturing process. In addition, using higher switching voltage to address the LCDs increases the power consumption.
The foregoing problems can be addressed through the use of liquid crystal devices comprised of composite materials which are mixtures of polymer and liquid crystals and which have suitable sub-millisecond response time for the higher demands of next generation viewing. Further, these devices display two modes of switching: phase-only (in-plane switching at low field) and birefringence (out-of plane at high field). Apart from being useful for flat panel displays, the devices are useful as spatial light modulators for applications such as optical waveguides, optical beam scanners, computer-generated holograms, and adaptive optics.
In accordance with the present invention, these novel electro-optical devices use the flexoelectric-optical effect of short-pitch cholesterics. By using a polymer to stabilize or modify the liquid crystal phase structures, two modes of electro-optical switching are possible: 1) at a low applied voltage the molecules rotate in the plane parallel to the plane of the substrates and can give high modulation of the transmitted light intensity due to the field-induced tilt of the optical axis; 2) at a high applied electric field the molecules are switched in the direction perpendicular to the plane of the substrates. The polymer content in the composite can vary from a few percent (polymer stabilization type) to as high as possible, such as 40%, preferably 20%, and more preferably 1 to 10% (polymer dispersed type) as long as the polymer does not interfere the switching of liquid crystal molecules.
It is known that a short-pitch cholesteric material with its helical axis originally oriented perpendicular to the plane of the substrates can be re-oriented to parallel to the substrate by using a small bias voltage. The literature reports the preparation of a cholesteric liquid crystal (ChLC) cell flexoelectro-optical device using in-plane uniform lying helical (ULH) texture. The uniform cholesteric film behaves as a switchable birefringent plate. An in-plane rotation of the optical axis can be achieved by applying an electric field across the cholesteric film, which was reported by Meyer as the flexoelectric effect. The term “flexoelectric effect” is used herein to refer to this in-plane rotation of the helical optical axis of a cholesteric film through the application of an electric field. The degree of field-induced in-plane rotation of the helical axis depends on the strength of applied electric field for a cholesteric material with positive dielectric anisotropy. The intensity of light transmitted through the cell as a function of applied electric field is governed by the following equation:I=Io sin22φ sin2(πdΔnIλ)where φ is the angle between the optical (helical) axis and the polarizer at zero field, d is the cell gap, Δn is the effect sample birefringence, and λ is the wavelength of light. Therefore, the linear modulation of the transmitted light incurred by the flexoelectric effect will be the major contribution at the low fields. An out-of-plane switching can be effected through the use of a positive dielectric anisotropy liquid crystal material and the application of an applied voltage which exceeds the critical field of unwinding the cholesteric helix. Unwinding of the helix takes place when a high voltage is applied to the device since the dielectric coupling is a polar and a linear effect. However, the flexoelectric-optical effect vanishes upon shutting down the device and therefore requires a warm-up period to achieve the threshold voltage.
There has been an attempt by P. Rudquist, L. Komitov and S. T. Lagerwall in 1998 and reported in Liq. Cryst., 24, at page 329 to stabilize the ULH texture of a short pitch cholesteric by polymer network created in the volume of the cholesteric. The authors indicated that once the field is removed, the cholesteric twists up again and the uniform laying helix (ULH) texture is reacted just as it was before unwinding. However, the stabilization of ULH texture by polymer network proposed by Rudquist et al resulted in a residual birefringence of the completely unwound cholesteric texture due to the morphology of the polymeric network formed into the bulk of the cholesteric liquid crystal by means of photopolymerisation of 10 wt % photoreactive monomer dissolved in the cholesteric liquid crystal. The residual birefringence in turn decreased the contrast of the cell optical appearance at applied electric field higher than the threshold one for unwinding of helical axis thus affecting severely the efficiency of the modulation in the cell birefringence due to the field induced helix unwinding. Moreover, the dense polymer network stabilizing the ULH texture of the cholesteric reported by Rudquist et al negatively affects the cell performance in the degree of the light intensity modulation as well as the switching time.
To our best knowledge an efficient polymer stabilization of short pitch cholesteric liquid crystal in ULH texture that avoids any residual birefringence originating from the polymer network created in the volume of the cholesteric liquid crystal oriented in ULH texture has not been realized yet. Most important, surface stabilization of the ULH texture of short pitch cholesteric has never been reported. In addition, device that operates in polar (flexoelectric) as well as quadratic, with the applied electric field (helix unwinding), mode has not been realized yet. The present invention relates to the discovery that a surface-localized and periodically-structured polymer can be used to create uniform lying helical texture with high optical contrast of flexoelectric cholesteric devices without the need for a threshold (or warm-up) voltage to maintain the uniform lying helical texture. The liquid crystal device of the present invention employs in-plane rotation of the optical axis which is achieved by applying an electric field across the cholesteric film to provide a flexoelectric effect and which enables two modes of switching. However, the out-of-plane dielectric coupling hinders the large in-plane rotation of axis from minimum to the maximum light transmittance and hence, the optical contrast. Therefore, as a further aspect of the invention novel cholesteric liquid crystal material and electro-optical optimization are used to achieve the maximum optical contrast for the in-plane switching mode without the dielectric coupled out-of-plane switching.